Matched Index of Refraction Technique
The benefit of the matched index-of-refraction technique is that it permits optical measurements to determine flow phenomena in passages and around complicated objects without locating a disturbing transducer in the flow field and without distortion of the optical paths. With a transparent model of different refractive index than the working fluid, the optical rays can be refracted in such a manner that measurements are either impossible or require extensive, difficult calibrations. The innovative advantage of the INL MIR system is its large size, leading to improved spatial and temporal resolution compared to others. Also, by using models fabricated from fused quartz and light mineral oil as flow medium, the quartz models are invisible in the visible spectrum at some wavelengths which provides refraction-free use of optical measurement systems. Indices in the MIR flow system are typically matched at 488 nm and 514 nm for laser Doppler velocimetry and at 532 nm for particle image velocimetry.